Mating behavior, movement influence disease dynamics

Reducing contact rates among wild boar groups may be more effective than reducing population densities at limiting disease spread in wildlife.

Aug 19, 2019

Disease outbreaks in wildlife often also affect farm animals and people, but the causes and dynamics of their spread are often complex and not well understood, according to an announcement from Forschungsverbund Berlin e.V. (FVB), a research institution in Berlin, Germany.

FVB said a team of scientists led by the Leibniz Institute for Zoo & Wildlife Research (Leibniz-IZW) has carried out an analysis of long-term data from an outbreak of classical swine fever in wild boars in the German federal state of Mecklenburg-Vorpommern that occurred between 1993 and 2000.

The results suggest that non-infected regions have a higher risk of infection due to changes in movement patterns, particularly during the mating seasons (autumn and winter), thus highlighting the importance of focusing intervention efforts on specific individuals, seasons and areas in the event of future outbreaks, the researchers said.

The findings were published in the Journal of Animal Ecology. The study was conducted by a team of scientists from the Helmholtz Centre for Environmental Research (UFZ), the Friedrich Loeffler Institute and the University of Potsdam under the direction of Leibniz-IZW.

"Studies such as these help us to uncover the temporal and spatial dynamics of diseases such as classical swine fever and to use these findings to derive possible causes for long-lasting epidemics as well as measures to prevent new infections and outbreaks," first author Cédric Scherer with Leibniz-IZW explained. The seasonal patterns of disease spread varied dramatically.

"Interestingly, at the county level, infection was more likely to occur in autumn and winter, while individual wild boars, especially the young, are most likely to become infected in spring during birth season," reported Stephanie Kramer-Schadt, who headed the Leibniz-IZW project. "We assume that this is due to the increased movement activity in autumn and winter. In particular, the search for mating partners and the shortage of food lead to more frequent changes of location and, thus, likely enable the spread of the disease beyond district boundaries."

Contrary to common interpretations, the density of wild boars in a municipality was not decisive.

"This finding is understandable, as almost all districts have more wild boar than necessary for the spread of infectious diseases," UFZ epidemiologist Hans-Herrmann Thulke said.

The detailed long-term data collected by the authorities in Mecklenburg-Vorpommern during the outbreak made it possible to investigate the temporal and spatial differences in the spread of the disease. The researchers analyzed the data for different phases of disease spread for individual animals as well as for entire municipalities.

Classical swine fever (also known as European swine fever) is a viral infection affecting wild and domesticated pigs. Despite similar symptoms, the pathogens responsible for classical swine fever and African swine fever are not related in the course of the disease.

Long-lasting outbreaks of classical swine fever among wild boars often lead to the infection spreading to agricultural pig farms, FVB said. This can cause considerable economic damage if millions of domesticated pigs are slaughtered in an emergency and export bans are imposed on pork products.

In order to limit the spread of classical swine fever in a wildlife population, vaccination baits could be used, and/or the density of wild boars could be reduced by hunting, the researchers suggested. Although lowering the density to a theoretical minimum was often discussed as a measure, this study showed that in later phases of an outbreak, it was not the density but the contact rates that likely increased due to changes in movement behavior and made it possible for the disease to persist for several years.

In order to prevent such persistence or future outbreaks, the focus should, therefore, be on reducing the contact rates among wild boar groups, the researchers concluded.